Unlike servers with power-down modes (e.g., sleep mode, hibernation mode) such energy-saving modes are not currently used for storage subsystems. The typical storage subsystem powers on all disk drives for all the time, even those drives which are practically never used. Spinning drives is quite costly in terms of energy consumption, hence lots of cooling is needed. There have been initiatives targeting data centers as well as individual machines for lowering power consumption, cooling effort, and carbon footprint, as the market for “green” initiatives in IT is growing with high customer interest.
In a disk system according to prior art such as IBM® DS8000®, IBM® DS6000® and IBM® DS4000® comprise a plurality of disk drives and disk controllers. Disk drives are arranged in arrays (such as RAID-5, RAID-10, RAID-6) to improve the availability. One array usually comprises 4-16 disks. The arrays in such disk systems have different purposes: some are used to store data, some are not yet configured for I/O and some are so called onDemand arrays which can be enabled by the customer using a feature code. According to these different purposes of arrays different power management policies can be defined and implemented.
Reducing the velocity versus stopping the disk balances the power saving and the access time. For example, a disk running at 60% of its nominal speed requires 7 seconds to become ready whereas a disk which is stopped requires 15 seconds. At the same time, a disk running at 60% of its nominal speed saves up to 51% power and a disk which is stopped saves 89% of it nominal power.